In the nuclear power industry, storage and transportation of nuclear fuel assemblies is complicated by having to "nuclearly" isolate portions of nuclear fuel from other portions in order to avoid conditions of criticality which could lead to an uncontrolled nuclear chain reaction. Nuclear fuel assemblies are made up of fuel rods supported in a rack structure and separated with grid spacers. Interspersed within the fuel rod matrix are control rod guide tubes.
Nuclear isolation in storage and transportation is presently done in three ways; (1) placing neutron absorbing material between nuclear fuel assemblies, or by placing nuclear fuel assemblies into neutron absorbing baskets, (2) dismantling the nuclear fuel assembly and consolidating fuel rods into close packed arrays, and (3) relying on "burnup" or depletion of fuel to allow close packing of nuclear fuel assemblies without interposing neutron absorbing material.
Each method has inherent disadvantages. Placement of neutron absorbing material between nuclear fuel assemblies or placement of nuclear fuel assemblies into baskets increases the volume required for storage or transportation of multiple nuclear fuel assemblies. Moreover, use of a basket imposes a fixed amount of neutron absorbing material whether the assembly is fresh or spent. Consolidation of nuclear fuel assemblies is limited because the operation must be done under water such as in a spent fuel pool. Relying on burnup precludes close storage or transportation of new or partially spent nuclear fuel assemblies. In addition, there is a risk that a spent nuclear fuel assembly is not as "spent" as expected.
It would be advantageous to closely pack nuclear fuel assemblies by eliminating neutron absorbing material placed between nuclear fuel assemblies, without having to consolidate them and using only enough neutron absorbing material appropriate for the status of a particular nuclear fuel assembly. The present invention offers just such a solution.